1. Field of the Invention
The present invention relates to an optical recording medium on which recording and reproduction are performed by using an optical head which carries a solid immersion lens. In particular, the present invention especially relates to an optical recording medium and an optical recording apparatus for the same which improve the durable performance for sliding movement effected between an optical head which carries a solid immersion lens and the optical recording medium, making it possible to record information at a high density by utilizing evanescent light, and reproduce the recorded information with high C/N.
2. Description of Related Art
Recently, the optical recording apparatus, which is an information-recording apparatus capable of recording a large capacity of data at a high density and quickly reproducing recorded data, is used in response to the development of the information-recording apparatus to conform to the multimedia. The optical recording medium, which is subjected to recording or reproduction by using the optical recording apparatus, includes the read-only disk such as CD and laser disks in which information is permanently stored by stamping the disk surface to have a concave-convex configuration upon production of the disk, the write-once type disk such as CD-R which are capable of recording only once, and the rewritable type disk in which data can be rewritten and erased any number of times by using the magneto-optical recording system or the phase-change recording system. When the information is recorded or reproduced on the optical recording medium by using the optical recording apparatus, a light spot is used, which is obtained by focusing a laser beam up to the diffraction limit by using a lens. The size d of the light spot is represented by d=xcex/NA provided that the wavelength of the laser is xcex, and the numerical aperture of the lens is NA (xe2x80x9cFoundation and Application of Optical Disk Storagexe2x80x9d, edited by Yoshito Tsunoda, Incorporated Association of Society of Electronic Information and Communication (1995), p. 65).
In order to record information on the optical recording medium at a higher density, it is necessary to decrease the recording laser spot size so that minute pits and magnetic marks are formed. However, according to the expression described above, in order to decrease the light spot, the laser wavelength (xcex) may be decreased, or the numerical aperture (NA) of the lens is increased. The semiconductor laser for performing reproduction on the optical disk used at present has the wavelength which is mainly 780 to 680 nm. A laser of orange color of 650 nm, which is shorter in wavelength than the above, begins to be used, for example, for the digital versatile disk (DVD-ROM). However, the short wavelength laser, which emits green or blue light of a wavelength shorter than the wavelength of the orange laser, is still under development. There is a limit to decrease the spot size by decreasing the laser wavelength.
On the other hand, as shown in FIG. 1, the numerical aperture (NA) of the lens is represented by NA=sin xcfx86 provided that the focusing half angle of the lens is xcfx86. The numerical aperture NA has a value smaller than 1. The lens, which is used at present, has NA of about 0.5. Even if NA=0.9, which approximates to the theoretical limit, is achieved, the laser spot size can be merely reduced to be 1/1.8 at most. On the other hand, if NA is increased, then the depth of focus of the lens system becomes shallow, and a problem arises in that a complicated control system should be used to maintain the focal point on the recording plane. Therefore, it is impossible to excessively increase NA. In the case of an ordinary optical recording apparatus, a lens approximately having NA=0.6 is used at the maximum.
A method for effectively increasing NA of the lens has been suggested, in which a solid immersion lens is used in order to decrease the spot size of the laser beam (Nikkei Electronics, No. 686, pp. 13-14, 1997.4.7). As shown in FIG. 2A, when a hemispherical solid immersion lens is used, and the laser beam is allowed to come perpendicularly into the lens surface, then the equivalent NA of the optical system is represented by nxc3x97NA provided that the refractive index of the solid immersion lens is n. As shown in FIG. 2B, when a super spherical solid immersion lens is used, and the laser beam is allowed to come so that the focal point is formed on the bottom surface of the super spherical lens, then the equivalent NA is represented by n2xc3x97NA. When the solid immersion lens is made of glass, the refractive index of glass is about 1.8. Therefore, the spot size can be decreased to be 1/1.8 when the hemispherical solid immersion lens is used, and the spot size can be decreased to be 1/3.2 when the super spherical solid immersion lens is used, respectively as compared with the case in which an ordinary objective lens is used.
When the solid immersion lens is used, the evanescent light, which leaks out from the solid immersion lens, can be used to perform recording and reproduction. The attenuation distance of the evanescent light is not more than the wavelength of the light emitted from the light source. Therefore, it is necessary to allow the solid immersion lens to approach the medium so that the solid immersion lens is disposed within the attenuation distance of the evanescent light. For this reason, when the solid immersion lens and the evanescent light are used in combination, it is necessary to use a flying type slider as used for a magnetic head of a fixed type magnetic disk (hard disk). FIG. 3 shows an example of the structure of the optical head for the magneto-optical recording medium based on the use of such a flying type slider. The optical head comprises an objective lens 71, a solid immersion lens 100, and a recording magnetic field-generating coil 104 incorporated into a flying type slider 102.
In the case of an ordinary magneto-optical recording apparatus, the light is radiated onto the recording layer through a transparent substrate of the magneto-optical recording medium. However, in the case of the optical head based on the use of the solid immersion lens, the spacing distance between the solid immersion lens and the optical recording medium is restricted as described above. Therefore, the magneto-optical recording medium adopts a structure in which a reflective layer, a second dielectric layer, a magneto-optical recording layer, and a first dielectric layer are stacked in this order on a substrate. It is necessary for the magneto-optical recording medium to be irradiated with the recording and reproducing light beam from the side opposite to the substrate, i.e., from the side of the first dielectric layer. For example, U.S. Pat. No. 5,202,880 discloses an optical recording medium and a recording and reproducing apparatus of the type in which the recording and reproducing light beam comes from the side opposite to the substrate.
In the case of the ordinary optical disk, in order to protect the recording layer, a protective layer is formed on the side of the recording layer surface opposite to the. substrate by applying ultraviolet-curable resin or Si resin curable in the atmospheric air. The protective layer generally has a thickness of several xcexcm to several tens of xcexcm. In the case of the system based on the use of the evanescent light by the aid of the solid immersion lens, it is impossible to form the protective layer made of resin on the second dielectric layer, because the protective layer made of resin is thicker than the attenuation distance of the evanescent light. Therefore, in this system, the recording and reproducing optical head is moved at a position separated by about 100 nm from the first dielectric layer as the uppermost layer, in the same manner as in the fixed type magnetic disk apparatus. For this reason, if the optical head varies its flying posture during the movement, then the optical head contacts with the second dielectric layer, and its surface is scratched in some cases.
The first dielectric layer, which is disposed at the uppermost layer of the magneto-optical recording medium, is formed of a hard material such as silicon nitride, silicon oxide, aluminum nitride, and silicon carbide. The film thickness thereof is 50 to 100 nm which is two to five times thicker than that used in the magnetic disk. The flying height of the recording and reproducing optical head can be 100 to 150 nm which is about two to three times higher than that used in the magnetic disk, when the evanescent light is used by the aid of the solid immersion lens. Therefore, the scratch, which is formed on the surface of the dielectric layer due to irregular sliding movement caused by variation of the posture of the optical head, does not become so deep to arrive at the recording layer. In many cases, the scratch is formed in the traveling direction of the optical head, as a grazed stripe-shaped scratch having a width of about several xcexcm to several hundreds of xcexcm and a depth of about several tens nm. In the case of the optical disk such as an ordinary magneto-optical disk, the laser spot size is about 1 mm at the minimum on the surface of the substrate, because the reproduction is performed through the transparent substrate. Therefore, the scratch of about several xcexcm to several tens of xcexcm formed on the substrate surface scarcely causes problems concerning recording and reproduction. However, the system, which is based on the use of the evanescent light by the aid of the solid immersion lens, utilizes the leakage of the laser beam having been focused up to the diffraction limit. Therefore, a problem arises in that the scratch, which merely has a width of several xcexcm formed on the surface of the dielectric layer, tends to cause any reproduction error due to variation of the amount of reflected light, i.e., the amount of reproducing light, caused by the interference at the edge of the scratch.
Further, there has been also a problem that when recording or reproduction is performed by means of the evanescent light by using the solid immersion lens, then the structure of the optical recording medium should be properly adjusted, and the positional relationship between the solid immersion lens and the optical recording medium should be properly adjusted in accordance therewith, because the attenuation distance of the evanescent light is extremely short as described above.
Moreover, if there is any irregularity on the surface of the optical recording medium, the flying height of the optical head supported by the flying type slider varies depending thereon. However, if the flying height of the optical head varies, any multiple interference of light may occur depending on the flying amount, in the air layer intervening between the surface of the optical recording medium and the light-emitting plane of the optical element installed to the optical head. If such interference is caused, the intensity of reflected light changes. The change in intensity of reflected light causes variation in reproduced signal output from the optical recording medium, possibly resulting in reproduction error.
The present invention has been made in order to dissolve the problems of the conventional technique described above, an object of which is to provide an optical recording medium in which the scratch is hardly formed on the recording medium surface due to collision between the optical head and the recording medium, and even if any scratch is formed on the medium surface, the scratch is in a degree not to cause any error during reproduction.
Another object of the present invention is to provide an optical recording medium in which the structure of the medium and the positional relationship between a solid immersion lens and the medium are properly adjusted in order to appropriately perform recording or reproduction of information by utilizing the evanescent light.
Still another object of the present invention is to provide an optical recording medium which makes it possible to suppress variation in reproduced signal intensity resulting from any influence of interference of light caused in a light-transmissive medium existing between the optical recording medium and an optical head.
Still another object of the present invention is to provide an optical recording apparatus in which an optical system is property adjusted in conformity with the optical recording medium in order to appropriately perform recording or reproduction by utilizing the evanescent light.
According to a first aspect of the present invention, there is provided an optical recording medium comprising, on a substrate, at least a reflective layer, a recording layer, and a dielectric layer, wherein:
a solid protective layer having a self-lubricating property is formed on the dielectric layer; and
evanescent light is allowed to come into a side of the solid protective layer having the self-lubricating property by using an optical head including a solid immersion lens carried thereon so that at least one of recording and reproduction information is performed.
The optical recording medium of the present invention may have the structure comprising at least the reflective layer, the recording layer, and the dielectric layer on the substrate. The solid protective layer having the self-lubricating property is stacked on the dielectric layer which is opposed to the optical head. Accordingly, even if the flying position of the optical head is varied, and the optical head slides on the surface of the optical recording medium, then the optical head smoothly glides on the surface of the recording medium. Accordingly, the optical head is not caught on the surface, and the sliding scratch hardly occurs. The protective layer having the self-lubricating property tends to be peeled off in a layered manner. Therefore, even when the optical head intensely collides with (slides on) the recording medium, the protective layer is firstly peeled off in the layered manner. Thus, the optical head and the dielectric layer are prevented from occurrence of sharp scratches. Therefore, it is possible to suppress the error and the defect of the reproduced signal which would be otherwise caused by the scratch formed on the surface on the side of being irradiated with the reproducing light beam on the optical recording medium.
In the present invention, the term xe2x80x9chaving a self-lubricating propertyxe2x80x9d means the fact that the material itself has the lubricating property by itself, for example, as in graphite and molybdenum disulfide. Those usable as the substance for forming the protective layer having the self-lubricating property formed on the optical recording medium according to the present invention include, for example, carbon film, inorganic substances such as molybdenum disulfide, lead oxide, cadmium oxide, and boron oxide, and high molecular weight compounds such as polytetrafluoroethylene, polyethylene, and nylon. Especially, the protective layer is desirably those in which the film is easily formed by means of the vacuum film formation method based on the physical technique, because the recording layer of the optical recording medium is usually formed by means of the vacuum film formation method based on the physical technique such as sputtering. It is desirable that the protective layer transmits the laser beam without attenuation during recording and reproduction. Therefore, it is preferable to use a carbon film or a diamond-like carbon film having a small extinction coefficient.
The optical recording medium of the present invention is subjected to recording or reproduction by using the optical head which carries the solid immersion lens and utilizing the evanescent light which leaks out from the solid immersion lens. According to the present invention, the optical recording medium based on the use of the evanescent light is classified into the following two types of optical recording media depending on the transmitting path of the evanescent light. In the first type optical recording medium, the protective layer having the self-lubricating property, which satisfies the following conditional expression (1), is formed at the uppermost layer disposed on the side opposite to the substrate:
1xe2x89xa6n0 sin xcex8 less than nxe2x80x83xe2x80x83(1)
In this expression, no represents a refractive index of the solid immersion lens, n represents a refractive index of the protective layer having the self-lubricating property, and xcex8 represents an angle of incidence of light with respect to a light-emitting plane of the solid immersion lens. That is, as shown in FIGS. 2A and 2B, the light is totally reflected at the interface between the air layer and the light-emitting plane of the solid immersion lens, and the evanescent light leaks out from the light-emitting plane toward the air layer, by satisfying the relationship of 1xe2x89xa6n0 sin xcex8 defined in the foregoing conditional expression (1) when the light coming into the solid immersion lens forms the angle of incidence xcex8 with respect to the light-emitting plane of the solid immersion lens. The evanescent light is transmitted through the air layer, and it arrives at the surface of the protective layer of the optical recording medium. Since the foregoing conditional expression (1) of n0 sin xcex8 less than n is satisfied, the evanescent light, which has arrived at the surface of the protective layer, behaves as ordinary light, and it is transmitted through the protective layer. Accordingly, it is appropriate that the flying amount (thickness of the air layer) is within the attenuation distance of the evanescent light regardless of the film thickness of the protective layer.
The second type optical recording medium is an optical recording medium characterized in that the protective layer having the self-lubricating property, which satisfies the following conditional expressions (2) and (3), is formed at the uppermost layer disposed on the side opposite to the substrate:
nxe2x89xa6n0 sin xcex8xe2x80x83xe2x80x83(2)
txe2x89xa6(xcexxe2x88x924h)/4nxe2x80x83xe2x80x83(3)
In these expressions, n0 represents a refractive index of the solid immersion lens, n represents a refractive index of the protective layer having the self-lubricating property, xcex8 represents an angle of incidence of light with respect to a light-emitting plane of the solid immersion lens, xcex represents a wavelength of light radiated from a light source, and h represents a distance between the light-emitting plane of the solid immersion lens and the surface of the protective layer. The refractive index n of the protective layer having the self-lubricating property satisfies 1xe2x89xa6n. Therefore, concerning the foregoing conditional expression (2), 1xe2x89xa6n0 sin xcex8 is satisfied. Accordingly, as shown in FIGS. 2A and 2B, the light is totally reflected at the interface between the air layer and the light-emitting plane of the solid immersion lens, and the evanescent light leaks out from the light-emitting plane toward the air layer, when the light coming into the solid immersion lens forms the angle of incidence xcex8 with respect to the light-emitting plane of the solid immersion lens. The evanescent light is transmitted through the air layer, and it arrives at the surface of the protective layer of the optical recording medium. Since the foregoing conditional expression (2) of nxe2x89xa6n0 sin xcex8 is satisfied, the total reflection condition holds at the interface between the air layer and the surface of the protective layer. The evanescent light leaks out again from the interface between the air layer and the surface of the protective layer toward the protective layer. In order to transmit the evanescent light from the light-emitting plane of the solid immersion lens to the bottom surface of the protective layer, it is necessary that the optical path length therebetween (h+nt) is not more than xc2xc of the wavelength of the evanescent light. Since the second type optical recording medium satisfies the foregoing conditional expression (3), the evanescent light is transmitted through the protective layer, and it arrives at the bottom surface of the protective layer. Subsequently, the evanescent light behaves as an ordinary light, and it arrives at the recording layer to record or reproduce information.
As described above, when information is recorded or reproduced by using the solid immersion lens and the evanescent light,. it is possible to appropriately select the first type optical recording medium or the second type optical recording medium to be utilized, for example, depending on the refractive index of the solid immersion lens to be used and the material quality of the protective layer. For example, the super spherical solid immersion lens makes it possible to collect the light on the light-emitting plane at an angle larger than the angle of incidence of the light coming into the surface of the solid immersion lens, as compared with the hemispherical solid immersion lens. Accordingly, NA is further increased, and consequently the spot size is further decreased. Therefore, when the super spherical solid immersion lens is used, then minute recording marks can be formed, and they can be reproduced, as compared with the case in which the hemispherical solid immersion lens is used. Thus, the use of the super spherical solid immersion lens makes it possible to perform recording and reproduction at a higher density. However, the increase in angle of incidence xcex8 makes it difficult to select the material for the solid protective layer which satisfies the foregoing conditional expression (1). Therefore, the second type optical recording medium is preferred as an optical recording medium to perform recording and reproduction at a high density by using the super spherical solid immersion lens.
In the optical recording medium of the present invention, if the refractive index of the protective layer having the self-lubricating property is greatly different from the refractive index of the dielectric layer contacting with the protective layer, then the laser beam is reflected by the interface between the both, and the laser beam is not utilized effectively. In order to suppress the reflection of the laser beam at the interface, it is desirable that an absolute value of difference between the refractive index of the protective layer and the refractive index of the dielectric layer is within 0.5. It is desirable that an absolute value of difference between an extinction coefficient of the protective layer and an extinction coefficient of the dielectric layer is within 0.2 in order to suppress attenuation of the laser beam.
In the optical recording medium of the present invention, a lubricant layer may be further formed on the protective layer having the self-lubricating property, by reason of further improvement in sliding performance. For example, a layer composed of a silicon lubricant may be formed in a film thickness of 1 nm to 5 nm.
When a carbon film is used as the protective layer having the self-lubricating property in the optical recording medium of the present invention, it is preferable that the hardness and the optical characteristics of the film may be controlled by allowing the carbon film to contain, for example, hydrogen, nitrogen, and fluorine. It is desirable that the protective layer having the self-lubricating property has a film thickness of not less than 5 nm and within 50 nm. If the thickness is less than 5 nm, it might be difficult to obtain sufficient sliding characteristics. There is no upper limit for the film thickness of the protective layer concerning the sliding characteristics. However, since a thick film may cause optical loss, it is desirable that the film thickness of the protective layer having the self-lubricating property has an upper limit of 50 nm.
Upon reproduction on the optical recording medium, the reproducing light, which is emitted from the optical system of the optical head, for example, from the objective lens such as the solid immersion lens, is collected on the recording layer. The light-transmissive medium such as air exists within the depth of focus of the optical system, i.e., between the light-emitting plane of the lens and the surface of the optical recording medium. If the thickness of the light-transmissive medium is within a coherent distance of the reproducing light beam, the interference of light occurs in some cases at the interfaces on both sides of the light-transmissive medium in accordance with the relationship between the thickness of the light-transmissive medium and the wavelength of the reproducing light beam. In the case of the conventional type optical recording medium in which the reproducing light beam comes into the side of the transparent substrate, no light-transmissive medium exists in the gap between the optical head and the substrate within the depth of focus of the optical system of the optical head. Therefore, no problem arises concerning the interference of light at the interface of the light-transmissive medium layer. On the other hand, when the flying type optical head is used to perform reproduction on the optical recording medium of the type in which the reproducing light beam comes into the side opposite to the substrate as in the first and second types of the optical recording media as described above, then the interference effect on the reproducing light beam is conspicuous between the optical element carried on the flying type optical head and the surface of the optical recording medium (at the both interfaces of the light-transmissive medium) depending on the variation in flying amount of the flying type optical head, and the reproduced signal intensity is varied. Therefore, in the present invention, it is desirable to satisfy the following condition in order to suppress any variation in reproduced signal intensity, which would be otherwise caused by the interference of light in the light-transmissive medium (air) existing between the optical recording medium and the optical head. That is, when a light beam having a wavelength of 650 to 700 nm is used as the recording or reproducing light beam, then a carbon layer or a diamond-like carbon having a film thickness of 5 nm to 30 nm may be used for the protective layer having the self-lubricating property, and a material having a refractive index of 1.9 to 2.2 and a film thickness of 80 nm to 120 nm may be used for the dielectric layer. A stable reproduced signal can be detected by using the flying type optical head under the condition as described above.
The optical recording medium of the present invention is directed to any one of optical recording media including, for example, read-only optical recording media such as CD, CD-ROM, and DVD-ROM in which information is reproduced depending on the presence or absence of concave-convex pits and holes and the difference in reflectance between the crystal phase and the amorphous phase; write-once type optical recording media such as CD-R in which recording is performed by making holes with a laser beam in an organic dye layer and an inorganic layer composed of Te compound or the like; magneto-optical recording media which comprise a recording layer of an alloy layer composed of a transition metal and a rare earth metal such as TbFeCo and DyFeCo; and rewritable optical recording media such as phase-change optical recording media in which a recording layer composed of a Ge alloy, an In alloy or the like can be reversibly changed between the crystal phase and the amorphous phase by means of irradiation with light.
The substrate used in the optical recording medium of the present invention is composed of a resin such as polycarbonate, polyolefine, polymethyl acrylate, polystyrene, and nylon. Besides, it is possible to use a disk substrate made of glass, silicon, thermally oxidized silicon, or a metal such as Al and Ti.
According to a second aspect of the present invention, there is provided an optical recording apparatus comprising an optical head, for recording or reproducing information on an optical recording medium, wherein:
the optical head comprises a flying type slider and a solid immersion lens installed to the flying type slider;
the optical recording medium comprises, on a substrate, at least a reflective layer, a recording layer, and a dielectric layer,
a solid protective layer having a self-lubricating property is formed on the dielectric layer, and evanescent light is allowed to come into a side of the solid protective layer having the self-lubricating property by using the optical head so that at least one of recording and reproduction of information is performed on the optical recording medium; and
the following expression is satisfied:
1xe2x89xa6n0 sin xcex8 less than n
provided that n0 represents a refractive index of the solid immersion lens, xcex8 represents an angle of incidence of light with respect to a light-emitting plane of the solid immersion lens, and n represents a refractive index of the solid protective layer having the self-lubricating property.
In the optical recording apparatus according to the second aspect of the present invention, the refractive index n0 of the solid immersion lens and the angle of incidence xcex8 of light with respect to the light-emitting plane of the solid immersion lens are adjusted to satisfy the foregoing conditional expression (1). Accordingly, the optical recording apparatus makes it possible to appropriately perform recording or reproduction on the first type optical recording medium.
According to a third aspect of the present invention, there is provided an optical recording apparatus comprising an optical head, for recording or reproducing information on an optical recording medium, wherein:
the optical head comprises a flying type slider and a solid immersion lens installed to the flying type slider;
the optical recording medium comprises, on a substrate, at least a reflective layer, a recording layer, and a dielectric layer,
a solid protective layer having a self-lubricating property is formed on the dielectric layer, and evanescent light is allowed to come into a side of the solid protective layer having the self-lubricating property by using the optical head so that at least one of recording and reproduction of information is performed on the optical recording medium; and
the following expressions are satisfied:
nxe2x89xa6n0 sin xcex8, txe2x89xa6(xcexxe2x88x924h)/4n
provided that n0 represents a refractive index of the solid immersion lens, xcex8 represents an angle of incidence of light with respect to a light-emitting plane of the solid immersion lens, h represents a distance between the light-emitting plane of the solid immersion lens and a light-incoming surface of the solid protective layer having the self-lubricating property, xcex represents a wavelength of light, n represents a refractive index of the solid protective layer having the self-lubricating property, and t represents a film thickness of the solid protective layer having the self-lubricating property.
In the optical recording apparatus according to the third aspect of the present invention, the refractive index no of the solid immersion lens, the angle of incidence xcex8 of light with respect to the light-emitting plane of the solid immersion lens, the wavelength xcex of light, and the distance (flying height of the optical head) h between the light-emitting plane of the solid immersion lens and the light-incoming surface of the solid protective layer having the self-lubricating property are adjusted to satisfy the foregoing conditional expressions (2) and (3). Accordingly, the optical recording apparatus makes it possible to appropriately perform recording or reproduction on the second type optical recording medium.
In the optical recording apparatuses according to the second and third aspects of the present invention, it is preferable that the light, which is allowed to come into the solid immersion lens carried on the optical head, is linearly polarized in a direction parallel to a track of the optical recording medium. The light, which is polarized in the direction parallel to the track of the optical recording medium, has a long distance capable of evanescent transmission for the component of the obliquely incoming light in a direction of the track, as compared with the light polarized in a direction perpendicular to the track. Therefore, it is possible to increase the reproduced signal from minute marks recorded at a high density. This embodiment is especially preferred when reproduction is performed on a read-only optical recording medium and on a phase-change optical recording medium. In order to linearly polarize the light coming into the solid immersion lens in the direction parallel to the track of the optical recording medium, for example, it is possible to provide a polarizer in an optical path between the light source and the solid immersion lens, or it is possible to use a laser as a light source which emits a linearly polarized laser beam.
The optical recording apparatus of the present invention may be used as an apparatus for performing recording and reproduction on a magnet o-optical recording medium. In this embodiment, the optical head comprises a magnetic field-applying means such as a magnetic coil.
In the optical recording apparatus of the present invention, the solid immersion lens may be a hemispherical solid immersion lens or a super spherical solid immersion lens.
In the present invention, a solid protective layer having a self-lubricating property may be formed on a surface of the flying type slider opposing to the optical recording medium. Accordingly, even if the flying position of the optical head is varied, and the optical head contacts with the surface of the optical recording medium, then the optical head smoothly glides on the surface of the recording medium. Accordingly, the optical head is not caught on the surface, and the sliding scratch hardly occurs on the surface of the optical recording medium or on the sliding surface of the optical head. The protective layer having the self-lubricating property tends to be peeled off in a layered manner. Therefore, even when the optical head intensely slides on the recording medium, the protective layer is firstly peeled off in the layered manner. Thus, the optical head and the optical recording medium surface are prevented from occurrence of sharp scratches. Therefore, it is possible to decrease the error and the defect of the reproduced signal which would be otherwise caused by the scratch formed on the optical head and on the surface of the optical recording medium on the side of being irradiated with the reproducing light beam.
The term xe2x80x9coptical recording apparatusxe2x80x9d herein means an apparatus which has at least one of functions to perform recording and reproduction by using light. The term has a concept including recording apparatuses for performing only recording, reproducing apparatuses for performing reproduction on read-only optical recording media such as CD-ROM, and apparatuses for performing recording and reproduction on rewritable media such as magneto-optical recording media.